Alkyl benzenes are useful, for example as intermediates in the production of various end products. It has been known for decades that alkali metals, when reacted with alkyl benzenes, displace benzylic hydrogens. Chester E. Claff & Avery A. Morto, 20 J. Org. Chem. 440 (1955); Herman Pines & Norman C. Sih, 30 J. Org. Chem. 280 (1965); Schramm & Langlois, 82 J. Am. Chem. Soc'y 4912 (1960). The resulting alkyl benzene anion and alkali metal cation pair undergoes a reaction with olefins at high temperature to give alkylation products in which single or all saturated benzylic carbon atoms are alkylated in such a way as to replace single or all of the benzylic hydrogen atoms on a carbon atom with one aliphatic chain per benzylic hydrogen atom. Such reactions yield a variety of products depending on the number of saturated benzylic carbon atoms and the number of hydrogen atoms on the given benzylic carbon atom. In the commercial production of alkyl benzenes, high purity product is generally desired and byproducts must be removed.
Several patents and publications address issues related to providing suitable methods for commercial production of alkyl benzenes for example, U.S. Pat. No. 8,277,652 B2, U.S. Pat. No. 6,100,437, and U.S. Pat. No. 4,950,831.
The examples cited in U.S. Pat. No. 8,277,652 B2 and U.S. Pat. No. 6,100,437 employ sodium-potassium alloy as the catalyst. The catalyst, during activation step, gets melted but remains as a different phase than the alkyl benzene. This helps metalation of alkyl benzene. Also, the use of a small amount of water in the reaction is mentioned in this process. This operation poses difficulties as follows:                The small amount of catalyst causes inefficient distribution of the catalyst in the alkane phase.        High temperature promotes the tarry byproducts which cover the catalyst surface and the reaction is lowered.        Also, the use of sodium-potassium alloy in presence of water is hazardous and affects the process economics.        
A serious problem with the present alkali metal especially sodium-potassium alloy catalyzed alkylation reaction is the fact that an alpha carbon of the alkyl benzene anion can add to either carbon atom comprising the olefinic double bond thereby giving two alkylation products. Moderation of reaction conditions has proven to be effective in eliminating multiple alkylations.
Inability to conveniently improve upon addition selectivity, however, is a problem. In addition to multiple alkylation of the alkyl benzene, the alkali metal catalyst utilized in the alkylation reaction seems to promote insoluble tarry byproducts formation due to poly condensation or the alkali metal catalyzed polymerization reaction. Other byproducts are also formed in the alkali metal catalyzed reactions which are soluble in reaction medium offering darkened color to the reaction medium.
Yet another problem in the present alkali metal catalyzed reaction is the proper distribution of the alkali metal alloy in the reaction system. Reference may be made to U.S. Pat. No. 8,277,652 B2, wherein 0.5 wt % of the sodium-potassium alloy is used as the catalyst. The alloy is used in the neat form and it is highly difficult to disperse the small quantity of the catalyst uniformly throughout the reaction medium which is destroyed at the end of the reaction. In order for reaction efficiency, the alkyl benzene-potassium ion pair, formed in the reaction between the alkyl benzene and potassium, should be dispersed effectively in the reaction mixture to get good selectivity for mono-alkylation.
The alkyl benzene-potassium ion pair and the alkyl benzenes form immiscible phases and the malfunctioning of the catalyst distribution in the reaction media leads to the side product formation. Generally, the present system being practiced commercially uses small amount of tall oil and water to help the catalyst and associated catalytic species in emulsified phase in the reaction medium. The small amount of water used in the present commercial practice to emulsify the reaction medium poses a serious safety concern.
It is generally believed that the alkyl benzene-potassium ion pair that is a catalyst complex gets dispersed at the emulsified phase which also coats the alkali metal alloy, thus higher surface area is available for the reaction with an alkene. The formation of tars and other byproducts reduces reactant utilization significantly. The formation of tars and other byproducts has further commercial impact in that the alkyl benzene-potassium ion pair gets covered by the tarry product, which reduces the rate of formation of alkyl benzene though the catalyst is active. Additionally, the active catalyst covered by the tarry product gets destroyed at the end of production cycle by water. This puts severe pressure on ecology and the process economics.
Similarly, with the current alkali metal alloy system, in which the alkyl benzenes are isolated after aqueous work-up that takes place towards end of the reaction, water is charged followed by layer separation. This causes troublesome emulsion formation due to high alkalinity of reaction medium, and causes loss of costly alkali metal alloy: the heavy emulsion tends to lose the organic phase. This puts severe pressure on ecology and the process economics.
Schramm and Langlois (1960) presented a detailed study of the alkylation of toluene using propylene as the form of alkene in the presence of highly dispersed sodium or potassium or lithium metal and various activators, such as anthracene, fluorene, indene, cyclopentadiene, α-methyl pyridine, and the like, as the chain initiator and shown that the alkylation of alpha carbon atom takes place at 149° C. to 307° C.
Schramm and Langlois also presented a detailed study of the effects of alkali metals on the yield of byproducts, particularly isomers due to non-selective addition at the double bond at a wide range of temperatures. They observed a lower product-to-isomer ratio over a temperature range of 107° C. to 204° C. in the presence of potassium as the catalyst. Alternatively, when sodium was used instead of potassium, the product-to-isomer ratio was large. Also, their data showed that in the presence of potassium as the catalyst, the rate of product formation was higher than in presence of sodium as the catalyst.
Herman Pines and Norman C. Sih (1965) presented a detailed study of alkylation of toluene, ethyl benzene and isopropyl benzene, using isoprene as the form of alkene in presence of highly dispersed sodium or potassium metal and o-chlorotoluene as a chain initiator; and showed that the alkylation of alpha carbon atom takes place at 125° C. to 133° C. in the presence of an initiator such as o-chlorotoluene.
Also, in a current industrial practice, higher amounts of catalysts, such as potassium metal and sodium metal alloy, are used in the pre-alkylation stage that gets destroyed by putting water or alcohol at the end of the cycle.
Accordingly, there is a need to provide a process for preparation of n-propyl benzene that overcomes the drawbacks of the prior art.